The present invention relates to sintered materials based on Si.sub.3 N.sub.4/ BN and/or SiAlON/BN and also to processes for their production. In accordance with the state of the art, composite ceramics consisting of Si.sub.3 N.sub.4, BN and oxidic grain-boundary phase are produced from the binary powder components Si.sub.3 N.sub.4, BN and oxidic sintering aids at high temperatures between 1,600.degree. and 2,000.degree. C. by sintering without pressure, pressure sintering, hot pressing or hot isostatic pressing. However, this conventional powder-metallurgy production method has a number of disadvantages:
Owing to the foliated morphology of the BN the addition of hexagonal BN powder to Si.sub.3 N.sub.4 powder with oxidic sintering additives leads to impairment of the pressing characteristics of the Si.sub.3 N.sub.4 powder with the sintering additives. With increasing BN content the pressed articles show increased creep recovery after moulding and reduced densities in the pressed state.
Just small amounts of BN impair the sintering characteristics of the Si.sub.3 N.sub.4 and result in higher porosities in the composite ceramic. In order to be able to produce high-quality compact ceramics, expensive manufacturing processes such as hot pressing or hot isostatic pressing have to be adopted (JA-A 01 083 507, Asano et al, Taikabutsu Overseas, Vol. 11, 1991, No. 3, pages 3 to 11, and Doche et al, Key Engineering Materials, Vol. 89-91, 1994, pages 449 to 454, Switzerland).
Sintering without pressure or pressure sintering results in composites having low density and poor mechanical properties (Isomura et al, Kawasaki Steel Technical Report, No. 23, Oct. 1990, pages 5 to 11).
With regard to the physical properties of the composite ceramic it is of essential importance that the BN is distributed homogeneously within the Si.sub.3 N.sub.4 matrix. Homogeneous initial powder mixtures ale required for high-quality composites. But even with elaborate grinding procedures it is difficult to achieve sufficient homogeneity of the initial powder mixture.
It is furthermore known that, according to DE 4 107 108 A1, it is possible to produce monophase amorphous silicon boron nitride ceramic (Si.sub.3 B.sub.3 N.sub.7). Baldus et al. (Better Ceramics Through Chemistry V, Vol. 271, 1992, pp 821-826) describe the production of ceramic moulded articles produced from a powder mixture consisting of 90 wt-% Si.sub.3 B.sub.3 N.sub.7 powder, 5 wt-% Al.sub.2 O.sub.3 powder and 5 wt-% Y.sub.2 O.sub.3 powder by sintering without pressure. Prior to being ground the Si.sub.3 B.sub.3 N.sub.7 powder has a specific surface of 60 m.sup.2 /g. But ceramics produced in this way have a porosity of almost 40%, so that their use in applications with high mechanical loads is ruled out.